Field of the Invention
The present invention relates to an image forming apparatus configured to measure a measurement image formed on a sheet being conveyed.
Description of the Related Art
Among image forming apparatus, there is a high demand of direct image printers which do not require plates that are used in offset printing or the like. Further, the direct image printers are frequently used as the image forming apparatus in order to respond to a reduction in time required for printing, services to individual customers, printing of a larger number of copies, an environmental problem of paper disposal at the time of printing failure, and the like. Among the direct image printers, ink jet printers which have advantages in terms of price and are suitable for photographic printing, and electrophotographic printers which have high productivity and can obtain a finished work close to that of offset printing are adopted particularly frequently. Such image forming apparatus are desired to have stability in image density and gradation along with the increase in full-color printing.
In order to meet such demands, in U.S. Pat. No. 5,583,644 and Japanese Patent Application Laid-open No. 2013-228640, there is proposed a technology of optimally adjusting the image quality. Those related-art image forming apparatus are configured to form an image after determining image forming conditions, for example, γ correction, based on a density of an image formed on a photosensitive drum or a sheet, to thereby adjust and stabilize the quality of the image to be formed. The image forming conditions are determined when the environment of the image forming apparatus varies or after a predetermined number of images are formed.
When an image formed on a sheet is to be measured, the sheet subjected to image formation is conveyed to a measurement position for measuring the image density by measuring means. The measuring means measures the image from the sheet being conveyed. When the image on the sheet is measured under a state in which the sheet is vibrating (state in which sheet fluttering occurs), the measurement result may have a measurement error. In this case, the image forming conditions have an error.
When the sheet fluttering occurs, a distance between the measuring means and the sheet varies. It has been experimentally confirmed that, when the distance between the measuring means and the sheet varies by about 1 mm, the output value of the measuring means changes by about 15%. As described above, the change in distance between the measuring means and the sheet may cause reduction in accuracy of measuring the image by the measuring means. As a result, it may become difficult to accurately adjust the image quality.